Optionally do not generate forrest docs

Currently ant package requires generating Forrest docs, which in general is a good thing. However, this is problematic on private networks for two reasons:

1. Forrest may not be available on build machines. This can be worked around by checking in a forrest release (either the extracted sources, or extract the release on demand) as part of your build process. However, docs are not strictly required and in practice no one will look at the docs on a production machine.
2. Forrest expects a public internet connection. Forrest attempts to update its plugins from a public internet location, which causes the HCatalog build to fail.

For these reasons, I propose adding a -Dnoforrest ant property that disables building the forrest documentation. By default, it still builds the docs so most users will not notice. However, users attempting to build on a private network will not have to patch around doc generation.